Optoelectronic components often include a housing, either to protect the components from dust or dirt that might interfere with optical transmission to or from the components or to allow ease of interconnection to or from the optoelectronic components. Environmental protection from oxygen or water vapour is sometimes also needed in order to achieve a useful operating lifetime. Some optical transmitter devices are held within a housing in order to improve eye safety or so that other devices such as an optical fibre connector can readily be connected to the transmitter device.
One common type of housing is the TO-header style coaxial package. This consists of a cylindrical metal housing with an axial window on an upper surface of the housing, and a number of electrical connection pins extending away in an axial direction from the opposite lower surface of the housing.
The metal housing provides excellent protection for any optoelectronic components within the housing, but creates problems in making electrical connections to such components. A conventional way of making such connections is to provide glass-insulated metal feedthroughs in the base to isolate the connections from the housing. This creates a lack of design flexibility as all components must be at the same potential as the housing base, or else isolated from the housing base. If wire bonds are used, the connections may need to be relatively long to reach from a feedthrough pin to the required component. Such long connections can limit high frequency performance, for example performance in excess of about 2 GHz. It is also the case that conventional feedthroughs generally include metal pins formed from an alloy such as that sold under the trade mark Kovar™. Kovar™ is a trade mark of CSR Holdings, Inc., a subsidiary of Carpenter Technology Corporation. Kovar™ is a metal alloy whose thermal expansion matches that of glass/ceramic.
Unfortunately, such pins are not ideal for high speed interconnects due to their induction. Careful attention must be paid to the interconnection with external components or circuit boards in terms of the pin length, form and shape. These feedthroughs can be difficult to optimize for fixed impedance transmission lines.
The space available within the housing for components can also be limited owing to the large area taken up by the glass seals. The maximum number of feedthroughs is similarly limited.
It is also necessary with glass-insulated metal feedthroughs to avoid introducing mechanical stresses in the feedthrough that can induce cracking. Therefore, when an external connection pin is to be curved, for example at right angles, the curved section of the connection pin must be separated from the feedthrough by about 3 to 4 mm. This increases the total size of the optoelectronic package.
One way to deal with these problems is to mount components on a printed circuit board (pcb), and then mount the pcb inside the metal housing. This allows for easier electrical routing within the housing, but increases the component count and assembly procedures. There also remain issues associated with alignment and connection of this subassembly to the feedthroughs in the housing base. This approach does not address the problem of the limited maximum number of feedthroughs.